The present invention relates to the technical field of recovery (recycling) of carbon fibers from carbon fiber-containing plastics, in particular from carbon fiber-reinforced plastics (CFPs), preferably from carbon fiber-containing or carbon fiber-reinforced composites (composite materials).
In particular, the present invention relates to a process for recovering (recycling) carbon fibers from carbon fiber-containing plastics, in particular from carbon fiber-reinforced plastics (CFPs), preferably from carbon fiber-containing or carbon fiber-reinforced composites (composite materials), and also the recycled carbon fibers obtainable by this process and their use.
The present invention additionally relates to plastics, building materials or cement-containing systems which comprise recycled carbon fibers obtainable by the process of the invention, or which have been produced using recycled carbon fibers obtainable by the process of the invention.
Finally, the present invention relates to shaped bodies (e.g. components), molds and sheet-like materials (e.g. nonwovens), in particular in the form of composite materials or compounds which comprise recycled carbon fibers obtainable by the process of the invention or which have been produced using recycled carbon fibers obtainable by the process of the invention.
In general, carbon fiber-reinforced plastics (also known synonymously as CFPs) in which a multiplicity of carbon fibers are embedded, preferably in a plurality of layers, as reinforcement in a matrix such as plastic can be referred to as fiber-plastic composites. As polymer matrix, it is possible to use both thermosets such as epoxy resins, acrylates and polyurethanes and also thermoplastics such as acrylonitrile-butadiene-styrene (ABS), polyamide (PA), polylactate (PLA), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyether ether ketone (PEEK) and polyvinyl chloride (PVC). However, it is also possible to embed carbon fibers in a matrix composed of ceramic (also referred to synonymously as ceramic fiber composites) in order to obtain thermally very stable components such a brake disks.
Carbon fiber-reinforced plastics (CFPs) have a high strength and stiffness combined with a low weight and are preferably used in fields where high weight-specific strengths and stiffnesses are required. For example, CFPs are used in the aerospace industry, in the wind power industry, in vehicle construction or for sports equipment such as bicycle frames, speed skates, tennis rackets, sporting arrows and fishing rods. In building and construction, carbon fiber-reinforced plastics (CFPs) can be adhesively bonded in the form of lamellae on the surface of the component in order to reinforce constructions.
The strength and stiffness of materials or components produced from carbon fiber-reinforced plastics (CFPs) is generally, as in the case of other fiber-matrix composites, significantly higher in the fiber direction than transverse to the fiber direction. Thus, for example, the strength transverse to the carbon fibers can be lower than the strength of the matrix material used. In order to ensure a uniform strength and stiffness of the materials or components composed of CFPs in all directions in space, individual fiber layers are laid in various directions. For example, in the case of high-performance construction components, the fiber directions can be determined by means of computer calculations such as the classical laminate theory in order to achieve the prescribed strength and stiffness.
The primary carbon fibers (also referred to synonymously as virgin fibers) used in the production of CFPs are predominantly produced industrially from carbon-containing starting materials, in particular polyacrylonitrile (PAN), by stabilization reactions in air, subsequent pyrolysis in an inert atmosphere and subsequent graphitization. The stiffness and strength of the primary carbon fibers can be controlled in a targeted manner during the production process by means of the pretensioning and also the temperatures in the carbonization and graphitization, so that various fiber types are commercially available. Owing to their inexpensive production, HT fibers (high-tensile fibers) and IM fibers (intermediate modulus fibers) are predominantly used as primary carbon fibers. In order to improve the adhesion of the primary carbon fibers after graphitization, an oxidation of the surface of the primary carbon fibers can be carried out by means of an electrochemical treatment. In general, the primary carbon fibers are subsequently provided with a size such as an epoxy resin and collected together to form rovings. These rovings are wound up onto conventional textile spindles in a last step.
Depending on the length of the primary carbon fibers used, various processes can be used for producing carbon fiber-reinforced plastics (CFPs). CFP parts having long primary carbon fibers can generally be produced by means of resin injection processes (also referred to as resin transfer molding (RTM)). In a first step of the resin injection process, preforms which consist of one layer or a plurality of layers of woven primary carbon fibers in order to ensure constant strength and stiffness in all directions in space are produced. These preforms are, in a second step, admixed in a closed casting mold with a liquefied matrix composed of plastic and optionally hardener. After curing of the matrix and removal of excess edge material, the corresponding CFP components are obtained.
The production of carbon fiber-reinforced plastics (CFPs) having short primary carbon fibers, in particular chopped primary carbon fibers, is generally carried out by means of injection molding. For this purpose, the chopped primary carbon fibers are mixed batchwise with a liquefied matrix composed of plastic(s), extruded and subsequently processed by means of injection molding to give CFP components.
However, the use of carbon fiber-reinforced plastics (CFPs) leads, in comparison with the use of similar components composed of light metals such as aluminum, magnesium and titanium, to considerably higher costs of the final product. This is related, in particular, to the complicated and costly production of primary carbon fibers from carbon-containing starting materials, in particular polyacrylonitrile (PAN). In addition, the worldwide consumption of primary carbon fibers for producing CFP components is increasing greatly, so that no significant reduction of the costs in the use of carbon fiber-reinforced plastics can be expected because of the high worldwide demand for primary carbon fibers.
Despite the high demand for primary carbon fibers, large quantities of primary carbon fibers which are unprocessed but have been preimpregnated with a plastic (referred to as prepregs or preimpregnated fibers), in which the plastic has been cured or which have exceeded the storage date, are disposed of as CFP-containing scrap.
In addition, large amounts of CFP-containing plastic scrap, which has to be disposed of, are obtained in the production of aircraft parts and parts for wind turbines and also as a result of modeling molds, production scrap, prototypes, incorrect batches and “end-of-life” components to be disposed of.
However, the disposal of CFP-containing plastics scrap in landfills is uneconomical because of the valuable carbon fibers present therein. Furthermore, it can generally be expected that the CFP-containing plastics scrap remains unchanged over a long period of time because of its chemical inertness and cannot be degraded in landfills. In addition, unlimited disposal of CFP-containing scrap is not readily possible or even prohibited because of legal requirements in many European countries.
There is therefore a great demand for inexpensive and efficient processes for recovering or recycling carbon fibers from CFP-containing scrap, in particular in the light of the worldwide demand for carbon fibers for the production of CFP components.
In the prior art, processes for recovering carbon fibers from CFP-containing scrap are known. However, these processes ensure the use of an inert atmosphere or the use of reduced pressure during the removal of the polymer matrix, and so sealed and complex devices and also complicated processes are necessary.
Due to the complex processes and devices, the costs for the recovery (recycling) of carbon fibers from CFP-containing scrap are high, with the processes described previously.
Furthermore, the CFP-containing scrap has to be pretreated in a complicated fashion, in particular by means of mechanical and/or chemical processes, before recovery (recycling).
For this reason, the use of recycled carbon fibers in CFP components has hitherto been possible to only a limited extent because of the mechanical pre-treatment, in particular the comminution. In addition, the recycled carbon fibers have a high proportion of pyrolysis and coking residues, which can have a negative effect on the incorporation into a polymer matrix.
Processes of this type in the prior art are described, for example, in DE 10 2008 002 846 B4, EP 0 636 428 A1 and DE 100 26 761 C1.
Processes for recycling carbon fibers from CFP-containing scrap on the laboratory scale are also known in the prior art. However, these processes are often complex and unsuitable for the recycling of carbon fibers on the industrial scale.